Stereoscopic dislay apparatus particularly useful with lcd projectors

ABSTRACT

Stereoscopic display apparatus, includes two projectors having inputs connectable to a source of digital data representing the color components of two stereoscopic images, and outputs outputting two optical beams each having a set of color components of different polarization states; a polarization preserving screen; an optical filter system using exclusively optical retarders for transforming the polarizations of the optical beams outputted by the two projectors into two color sets, in which all the color components of one set are polarized in one polarization state, and all the color components of the other set are polarized in an orthogonal polarization state; and means for stacking the two color sets onto the polarization preserving screen, such as to enable stereoscopic viewing of the two color sets via orthogonally polarized filters. Some of the described preferred embodiments involve switching of one color component, e.g., the green color component, at the inputs to the two projectors. In some described embodiments, the optical filter system outputs two beams stacked onto the screen, whereas in other described embodiments, the optical filter system produces a single output beam applied to the screen.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to stereoscopic display apparatus in whichthe stereo pair images are differentiated by their state ofpolarization. The invention is particularly useful with respect to LCD(liquid crystal display) projectors or projection engines, and istherefore described below in apparatus using such devices.

A stereoscopic display is made up from two superimposed images,representing the left-eye and the right-eye views, respectively. Inorder to create a stereoscopic effect, each eye of the viewer must beexposed only to its corresponding image. There are several known methodsto achieve this, one of them being known as a “passive” method. In thepassive method, the two stereoscopic images are polarized in mutuallyorthogonal polarization states. To view the display the observer wearsspecial binoculars with appropriate polarization filters, such that eachfilter transmits efficiently to each eye the light of one image andrejects the light of the other.

A block diagram of a passive stereoscopic projection system is shown inFIG. 1. The image generator provides the stereoscopic contents via twoelectronic signals to the two projectors. The beam of each projector ismanipulated by an appropriate optical filter to ensure that the twoimages that are displayed by the two projectors will be polarizedorthogonally to each other. The two filters can be disjoint or combinedin a single unit. Another possible design is shown in FIG. 2. In thisdesign the filters are disposed internally between the projection engineof the projector and the projection lens, and their output is combinedoptically. In this design there is only one projection lens. This factfacilitates the setup and the use of the projection unit.

Two types of polarization states are normally used in stereoscopicdisplays: linear and circular. Linear polarization is most popularbecause linear polarizers are most common and normally less expensivethan circular polarizers. However, filtering of linearly polarizedimages is sensitive to the viewer's head orientation. For this reasoncircular polarization is favored whenever the viewing is combined withmotion.

If the output light of the projector is unpolarized, it is possible tocreate two orthogonally polarized images simply by mounting linearpolarizers on the two projectors, oriented in such a manner that theangle between their polarization axes is 90°; see for example Andrew J.Woods, “Optimal Usage of LCD Projectors for Polarised StereoscopicProjection”, Proc. of SPIE Vol. 4297 (2001). This method is used withdigital-light-processing (DLP) projectors but involves a loss of atleast 50% of the light.

Many of the projectors used today are LCD (liquid crystal display)projectors. The output beam of LCD projectors is made up of three colorcomponents: red, blue and green. All three color components are linearlypolarized, but the polarization direction of the red and the bluecomponents is perpendicular to the polarization direction of the greencomponent, as illustrated in FIG. 3. Such a beam will be referred to as“cross-polarized”.

In general, when a polarizer is disposed in the output beam of an LCDprojector, it will seriously distort the colors of the image. Forinstance, if the polarizing axis is aligned parallel to the polarizationplane of the green light, all red and blue light will be absorbed, andonly the green content of the image will be transmitted. The only way topolarize linearly all three colors in the same direction whilepreserving their relative intensities is to align the polarizer at 45°with respect to all three colors. But this method also results in theloss of at least 50% of the light since the intensity of all threecolors will be reduced, equally, by at least 50%.

One can thus use the same method used to create mutually orthogonalimages with DLP projectors also with LCD projectors. The only differenceis the additional requirement to orient the polarization axes of thepolarizers at 45° with respect to the polarization directions of thecolor components. This is illustrated in FIG. 4, where the reducedlengths of the arrows of the various color components indicate the lightloss incurred by the use of polarizers.

LCD microdisplays, which serve as the image-generating element in LCDprojectors, need polarized light to function properly. As the lightsources used for these projectors (arc lamps) are unpolarized, much oftheir light is wasted and the optical efficiency is reduced. Thistempted practitioners in the field to invent LCD projectors that canfully utilize the unpolarized light of the source. Such projectors withtransmissive and reflective LCD microdisplays were described in Atarashiet al., U.S. Pat. No. 5,172,254 and Colucci et al., U.S. Pat. No.6,231,189, respectively. Both utilize six LCD microdisplays, two foreach color. Such projectors could be used for stereoscopic display, aseach can accept two full-color images. The present invention focuses onusing available off-the-shelf projectors or projection engines to avoidthe high cost involved in development of new designs such as thosedescribed in the patents cited above.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

One object of the present invention is to provide stereoscopic displayapparatus having advantages in the above respects. A more particularobject of the present invention is to provide stereoscopic displayapparatus which is optically efficient and which exhibits low cross talkbetween the left and right images.

According to a broad aspect of the present invention, there is providedstereoscopic display apparatus comprising: two projectors having inputsconnectable to a source of digital data representing the colorcomponents of two stereoscopic images, each of the projectors having anoutput outputting an optical beam having a set of color components inwhich one color component of the set is of an orthogonal polarizationstate with respect to the other color components of the set; apolarization preserving screen; an optical filter system usingexclusively optical retarders to manipulate the polarization states fortransforming the polarization states of the optical beams outputted bythe two projectors into two color sets in which all the color componentsof one set are polarized in one polarization state and all the colorcomponents of the other set are polarized in an orthogonal polarizationstate; and stacking means for stacking the two color sets onto thepolarization preserving screen, such as to enable stereoscopic viewingof the two color sets via orthogonally polarized filters.

Several embodiments of the invention are described below for purposes ofexample. These embodiments differ considerably in many respects. Theyhowever share one common feature: all transformations of the mainpolarization states are done exclusively with optical retarders. Thisfeature is fundamental to the present invention, as it allows the highoptical efficiency.

In one group of embodiments, the digital data representing one colorcomponent (e.g., the green color component) of the two stereoscopicimages is switched between the inputs of the two projectors. In theseembodiments, the optical filter system includes, at least onepolarization transformer that transforms each color component inputtedinto the respective projector from one polarization state to anotherpolarization state while preserving polarization orthogonality.

In some described embodiments, each of the polarization transformers isa half-wavelength retarder rotating the polarization direction of theoptical beam outputted by the respective projector; and in otherdescribed embodiments, each of the polarization transformers is aquarter-wavelength retarder converting the polarization of the opticalbeam outputted by the respective projector from linear to circular.

The switching of one color component at the input may be effected, e.g.,by crossing the wires carrying the respective color component signalsbetween the inputs to the two projectors, or by pre-processing thedigital data before inputted into them.

Several other embodiments of the invention are described below notinvolving switching of a color component of the original stereoscopicimages between the inputs to the two projectors. In these embodiments,the optical filter system includes for each projector a polarizationrectifier that transforms a plurality of color components in differentpolarization states at the input into the same polarization state at theoutput. In one described embodiment, each polarization rectifierseparates and processes the color components in two optical paths; andin another described embodiment, each polarization rectifier contains astack of optical retarders which align the polarizations of all thecolor components.

According to still further features in described preferred embodiments,the optical filter system may further include polarizing (eitherconventional or pleochroic) clean-up filters for increasing thepolarization ratio of the color components of the optical beamsoutputted from the two projectors, before stacking them on thepolarization preserving screen. Stacking of the output beams from theprojectors onto the polarization preserving screen may be effected byimage warping or by the use of optical beam splitters.

According to still further features in described preferred embodimentsthe optical filter system is disposed between the projection engines ofthe projectors and the projection lens, and a polarization beam splittercombines their output. In this embodiment the projection unit has onlyone projection lens, facilitating its use and setup.

According to another aspect of the invention, there is providedstereoscopic display apparatus comprising: two projection engines havinginputs connectable to a source of digital data representing the colorcomponents sets of two stereoscopic images, each of said projectorshaving an output outputting an optical beam having a set of colorcomponents in which at least one color component of the set is of anorthogonal polarization state with respect to the other color componentsof the set; a polarization preserving screen; a polarization rectifierfor each projector effective to manipulate said polarization statesexclusively by optical retarders, and to transform the beams outputtedby the projection engines to beams in which all color components havethe same polarization state in such a manner than the two transformedbeams have mutually orthogonal polarizations; a polarization beamsplinter for combining the transformed beams into one co-axial beam; anda projection lens for imaging the stereoscopic images on said screen.

As indicated earlier, the invention is particularly useful, and istherefore described below, with respect to LCD projectors outputting thered and blue color components in one polarization state, and the greencolor components in an orthogonal polarization state.

As will be described more particularly below, the foregoing features ofthe invention enable the construction of various types of stereoscopicdisplay apparatus which are optically efficient and which exhibit lowcross-talk between the left and right images.

Further features and advantages of the invention will be apparent fromthe description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of a passive stereoscopic projection withexternally disposed optical filters systems;

FIG. 2 is a block diagram of a projection system with internallydisposed optical filters systems and a single projection lens;

FIG. 3 illustrates the polarization directions of the color componentsin cross-polarized LCD projectors;

FIG. 4 diagrammatically illustrates the manner of creating two mutuallyorthogonal polarized beams in LCD projectors;

FIG. 5 is a block diagram illustrating the function of a polarizationtransformer;

FIG. 6 is a block diagram of a stereoscopic projection system using twopolarization transformers as optical filters with switching of the greencolor components at the input to the projectors;

FIG. 7 illustrates another manner of switching the green content bypre-processing the inputted data;

FIG. 8 is a block diagram of a stereoscopic projection system using twopolarization transformers as optical filters displaying a preprocessedcontent with green components switched;

FIG. 9 diagrammatically illustrates the manner in which two pleochroicpolarizers are used to clean up a cross polarized beam;

FIG. 10 shows a simple embodiment of the two polarization transformeroptical filter system;

FIG. 11 illustrates diagrammatically the function of a polarizationrectifier;

FIG. 12 is a block diagram of a stereoscopic projection system using twopolarization rectifiers as optical filters;

FIGS. 13 and 14 show preferred embodiments of a polarization rectifier;

FIG. 15 shows a complete assembly of a polarization rectifier with aclean-up polarizer;

FIG. 16 illustrates the evolution of the polarization states in the twoprojector beams subject to the filters such as shown in FIG. 15;

FIG. 17 shows the relative optical alignment of the three layers of thepolarization rectifier shown in FIG. 15;

FIG. 18 shows a stereoscopic projection system with a single projectionlens.

It is to be understood that the foregoing drawings, and the descriptionbelow, are provided primarily for purposes of facilitating understandingthe conceptual aspects of the invention and various possible embodimentsthereof, including what is presently considered to be a preferredembodiment. In the interest of clarity and brevity, no attempt is madeto provide more details than necessary to enable one skilled in the art,using routine skill and design, to understand and practice the describedinvention. It is to be further understood that the embodiments describedare for purposes of example only, and that the invention is capable ofbeing embodied in other forms and applications than described herein.

PRELIMINARY CONSIDERATIONS

General Requirements

Described below are several optical assemblies that can be used tocreate an optically efficient stereoscopic display in accordance withthe present invention. These assemblies are referred to as “opticalfilter systems”, or simply as “filters”.

The primary function of the filter is to create two mutually orthogonalRGB (Red, Green, Blue) sets that are used to display the pair ofstereoscopic images. In order to maintain low cost, the filtersdescribed here are designed for use with standard commercial LCDprojectors. Such projectors cannot be used to create a stereoscopicdisplay without a filter because they are polarized identically.

Another important factor of such filters is the filter opticaltransmission efficiency. The prior art polarizing filters used instereoscopic display apparatus generally have only 50% efficiency. Thefilters described here have much higher efficiencies, which is achievedby manipulating the intrinsic polarization properties of the projectors.

Most of the intensity of a polarized beam is located in one polarizationstate, referred to as the “main” state. In nature, perfect polarizationdoes not exist, and a certain amount of power of a polarized beam can bealways found in the polarization state that is orthogonal to the mainstate. The polarization quality of a polarized beam may be measured byits polarization ratio. This is the ratio between the intensities of themain state and the orthogonal to the main polarization components of thebeam. In passive stereoscopic displays, it is critical to have thepolarization ratio in each image in the stereoscopic pair as high aspossible. This is because the residual polarization components that areorthogonal to the main polarization direction cause cross talk betweenthe left and the right stereoscopic images. In other words, a smallamount of light from the right image reaches the left eye, and viceversa. This cross talk gives rise to a spurious effect, called“ghosting”, which degrades the quality of the display.

In a typical commercial projector, the polarization ratio of the colorcomponents may be too low for stereoscopic applications. Thus, theoutput beams should have a higher polarization ratio than the inputbeams. The operation of increasing the polarization ratio is referred toas “clean-up”. Special “clean-up” filters are described below reject thedestructive polarization components.

The filters must also preserve the colors and the uniformity of theoriginal images as much as possible.

The filters may be disposed externally to the projector, as shown inFIG. 1. This is the simplest utilization of filters because it does notrequire any modification in the projectors. However, in this arrangementthe user has to adjust the zoom and the focus of two separate projectionlenses, as well as correct relative image distortions caused by stackingtwo separate beams. This complicates the projection apparatus and itsuse.

The filters can be also disposed internally before the projection lens,as shown in FIG. 2. A projector without a projection lens is sometimesreferred to as “projection engine”. The FIG. 2 system has a singleprojection lens and is therefore more user-friendly. However, it is muchmore expensive to make such an apparatus as a special projection lensmust be developed for it.

A projector beam is comprised of three color components. Each colorcomponent has two attributes: (1) color (red, green, or blue); and (2)polarization (either horizontal or vertical). A filter performs twofunctions on each beam component:

(i) Manipulation of the component polarization state; and

(ii) Increase of the polarization ratio of the component (clean up).

Manipulation of the polarization state can be rotation (for linearpolarization), or conversion from linear to circular polarization.

The polarization states inputted to the filter are denoted by thesymbols α and β, and the output polarization states by γ and δ. Theoutput polarization states are different, in general, from the inputpolarization states. State α is orthogonal to state β, and state γ isorthogonal to state δ. There are no general orthogonality relationsbetween the input (α,β) states and the output (γ,δ) states. In thepreferred embodiments of the invention described below, the projectorsdetermine the input polarization states; in this case LCD projectors,which output the red and blue color components in one polarizationstate, and the green color component in an orthogonal polarizationstate, as illustrated in FIG. 3. The output polarization states arenormally designed to fit the standard polarization filters used instereoscopic glasses. In all the described preferred embodiments theoptical filter system transforms, by utilizing exclusively opticalretarders for polarization state manipulation, the optical beamsoutputted by the two projectors into two color sets in which all thecolor components of one set are polarized in one polarization state, andall the color components of the other set are polarized in an orthogonalpolarization state, such as to enable stereoscopic viewing of the twocolor sets via orthogonally polarized filters.

The images corresponding to the two output beams of the filter willappear in general displaced and distorted relative to each other on thescreen. The graphical contents of the output beams must be preciselystacked one on top of the other. This can be achieved either by opticalmeans using optical beam splitters, or by proper image warping (in thiscase, a projective transformation) applied to the corresponding graphicscontent. In many cases, with a proper mechanical alignment the necessaryimage warping can be reduced to an operation known as “keystonecorrection”. This image warping is a built-in feature in most modernprojectors. In general, stacking by image warping is more flexible andless expensive than optical stacking.

DESCRIPTION OF PREFERRED EMBODIMENTS

Described below are two types of filters: filters based on polarizationtransformers, and filters based on polarization rectifiers.

Filters Including Two Polarization Transformers

A block diagram of a polarization transformer is shown in FIG. 5. Thisis an optical device that accepts at input a cross-polarized beam inmutually orthogonal polarization states α and β, and outputs also across-polarized beam but in possibly different mutually orthogonalpolarization states γ and δ.

Of particular interest are polarization transformers with linearpolarizations in both input and output. In this case, the only thefunction of the polarization transformer is simply to rotate thepolarization directions of the input components by a certain angle. Thiscan be achieved by an optical element known as “half-wavelengthretarder”. Another polarization transformer that may be used is a linearto circular polarization converter. This transformation is accomplishedby an optical component known as “quarter-wavelength retarder”.

A block diagram of a filter based on polarization transformers is shownin FIG. 6. The two eye image sources are connected to the projectors insuch a manner that the green signal leads are crossed. Thus projector 1accepts the red and the blue components of the eye no. 1 image, and thegreen component of the eye no. 2 image. Correspondingly, projector 2accepts the red and the blue components of the eye no. 2 image, and thegreen component of the eye no. 1 image. The outputs of the twoprojectors are inputted to two different polarization transformers. Thetransformer of projector 1 transforms polarization state α topolarization state γ, and polarization state β to polarization state δ.The transformer of projector 2 transforms polarization state α topolarization state δ, and polarization state β to polarization state γ.The output beams of the filters are optionally cleaned up with specialpolarizers called “pleochroic”, whose function will be explained below.

The images created on the polarization-preserving screen scatter theirlight in all directions; so bare eyes can see both imagessimultaneously. To excite the 3D sensation it is necessary that each eyewill be exposed to its image only. This is achieved by disposingappropriate polarizing filters before the eyes. In the example shown inFIG. 6, the correct viewing conditions are created by disposing a γfilter in front of eye no. 1, and a δ filter in front of eye no. 2.

Another option for the utilization of this filter is to use apreprocessed stereoscopic content in which the green component wasswitched between the two images, as shown in FIG. 7. FIG. 7 (A) showsthe original content, and FIG. 7 (B) the same content with the greencomponent switched. The processed images shown in FIG. 7 (B) cannot beregarded as “left” and “right” images, as each image represents mixedcontent from both eyes. A block diagram of a polarization transformersfilter using mixed image sources is shown in FIG. 8. This scheme doesnot need crossing of the wires carrying the green signal, because thegreen content was switched already by content preprocessing.

This method is easier and less expensive to implement than the hardwarewire crossing. It is true that switching of the green content betweenthe two stereoscopic images requires computing resources. However,switching the green signals requires perfect synchronization between thetwo video channels. This is generally unavailable, except in high-endand expensive equipment.

FIG. 10 shows a particularly simple implementation of the polarizationtransformer filter. In this implementation it is assumed that thepolarization states α and β are linear (as is the case in practice). Thepolarization transformer in the projector 1 path is omitted. Thepolarization transformer in the projector 2 path is a simplehalf-wavelength retarder. It is aligned in such a manner that it inducespolarization states rotation by 90°. Thus polarization state α istransformed to polarization state β, and polarization state β istransformed to polarization state α. The reader can easily convincehimself that the desired polarization structure of the output beams isindeed achieved.

This simple filter is described here more for the sake of illustrationrather than as actual suggestion for commercialization. It has twodisadvantages: the treatment of the two channels is not symmetrical, andthe states α and β do not generally match the standard polarizedbinoculars used for stereoscopic viewing. Asymmetry in the two channelsmay create artifact differences between the left and the right images,and degrade the display quality.

Beams with well-defined polarization can be cleaned up with regularpolarizers, which are aligned in such a manner as to transmit the mainpolarization state of the beam while absorbing the residual radiation inthe polarization state that is orthogonal to it. Cross-polarized beamsdo not have a well-defined polarization, and cannot be cleaned up withregular polarizers. Special types of polarizers, called “pleochroic”polarizers, can be used to clean up cross-polarized beams.

A pleochroic polarizer is an optical filter that transmits all light inone state of polarization, and absorbs the orthogonal state ofpolarization in a chosen spectral band. Such a filter acts as apolarizer for the chosen spectral band, and as a transparent window forall other light. For instance, a pleochroic filter can be made thatabsorbs green light in one state of polarization, and is transparent toall other light. Such a filter is called “magenta pleochroic”. A filtercan also be made that absorbs red and blue light in one state ofpolarization, and is transparent to all other light. Such filter iscalled “green pleochroic”

If the pleochroic filter is designed for linear polarization states, itwill have a transmissive axis and an absorptive axis orthogonal to eachother.

To clean up the green color component in a cross-polarized beam, amagenta pleochroic filter can be used. If the transmissive axis of thisfilter is aligned with the direction of the green color component mainpolarization, this color component will be cleaned-up while the othercolor components will remain intact. Similarly, a green pleochroicfilter can be used to clean up the red and the blue components. Two suchfilters in series can clean up all three color components. Thisarrangement is shown in FIG. 9. The primed color components are colorcomponents that were “cleaned-up”, or, in other words, theirpolarization ratio has been increased. Such a clean-up filter made upfrom one or more pleochroic polarizers will be referred to as a“pleochroic clean-up filter”.

Normally, the green color component in projectors has much higherintensity than the other two color components. In addition, the humaneye response to the green color is higher than to the other colors. Thecombination of these facts makes the green color dominant in projectedimages. Therefore, cleaning the green color only may be sufficient inmany applications. Using only one pleochroic polarizer to clean up thebeam reduces cost and losses.

Filters Including Two Polarization Rectifiers

A polarization rectifier is an optical device that accepts two or morecolor components in different polarization states, and produces anoutput composed of all input components identically polarized. A blockdiagram of a polarization rectifier for a projector beam is shown inFIG. 11.

A block diagram of two polarization rectifiers filter is shown in FIG.12. The two images sources are coupled directly to the projectors. Adifferent polarization rectifier processes the output of each projector.The polarization rectifier of projector 1 transforms both the a and theβ polarization states to another polarization state γ, and hepolarization rectifier of projector 2 transforms both the α and the βpolarization states to yet another polarization state δ, in such amanner that γ and δ are mutually orthogonal. The output beams areoptionally cleaned up by regular polarizers. It is seen that the correctviewing conditions are created by disposing a y polarization filter infront of eye no. 1, and a δ polarization filter in front of eye no. 2.

A possible embodiment of a polarization rectifier is shown in FIG. 13.In this embodiment it is assumed that the input polarization states αand β are linear, and that so is the output polarization state γ too.

A green-reflecting dichroic mirror splits the polarization states of theinput beam. This mirror has the property that it reflects the greenlight, and transmits the red and the blue light. A half-wavelengthretarder rotates each one of the separated polarization states so thatboth acquire the same polarization state γ. The two color components,being now identically polarized are recombined by a red and bluereflecting dichroic mirror. The resultant output beam is made up of allthree components polarized in state γ. The direction of state γ can becontrolled by the orientations of the half-wavelength retarders.

Readers familiar with related optical techniques will recognize that apolarization beam splitter can replace the green-reflecting dichroicmirror at the input of the polarization rectifier. Also, when generalpolarization transformers are used instead of the half-wavelengthretarders, an arbitrary output polarization state γ can be achieved.

Another device that can serve as a polarization rectifier for an LCDprojector is a special stack of optical retarders as described, forexample, by Sharp, U.S. Pat. No. 6,310,673. Sharp teaches, among otherthings, how to make a filter that will rotate the green colorpolarization direction by 90°, while keeping intact the polarizationplane of the other color components. Such a filter has an axis, whichhas to be aligned with the red and blue colors polarization direction inorder to achieve the desired effect. When such a filter is mounted inthe proper orientation to receive a cross-polarized LCD beam, all colorswill emerge polarized in the same direction. Such green rotating filtersare manufactured by ColorLink (Boulder, Colo.) under the commercial nameColorSelect™ The ColorSelect™ filter alone cannot produce a generallinear output polarization state, like the filter shown in FIG. 13. Apolarization rectifier with an arbitrary linear output polarizationstate can be created by combining a ColorSelect™ filter with ahalf-wavelength retarder, as shown in FIG. 14.

FIG. 15 shows a compact polarization rectifier, which incorporates aclean-up filter. It is made of three layers: a green-rotatingColorSelect™ filter, a half-wavelength retarder, and a linear polarizer.The function of two such devices that can be used for a two polarizationrectifiers filter is illustrated in FIG. 16. FIG. 16 (A) shows theoriginal beams; FIG. 16 (B) shows the beams after the ColorSelect™filter; and FIG. 16 (C) shows the beams after the half-wavelengthretarder. The orientations of the three layers in both devices is shownin FIG. 17.

The two polarization rectifiers filter lends itself to an efficientcombining of its output beams by a polarization beam splitter. Apolarization beam splitter is a device that transmits one polarizationstate while reflecting the orthogonal polarization state. It can be usedto combine the two output beams of the filter into a single co-axialbeam because these beams have well defined and mutually orthogonalpolarizations. Combining the two outputs of the filter is particularlyuseful for creating a stereoscopic projection unit with a singleprojection lens, as shown in FIG. 18.

While the invention has been described with respect to several preferredembodiments, it will be appreciated that these are set forth merely forpurposes of example, and that many other variations, modifications andapplications of the invention may be made.

1-24. (canceled)
 25. Stereoscopic display apparatus comprising: twoprojectors having inputs connectable to a source of digital datarepresenting the color components sets of two stereoscopic images, eachof said projectors having an output outputting an optical beam having aset of color components in which at least one color component of the setis of an orthogonal polarization state with respect to the other colorcomponents of the set; a polarization preserving screen; an opticalfilter system using exclusively optical retarders to manipulate saidpolarization states for polarizing the output beams of the twoprojectors into desired mutually orthogonal polarization states;polarizing clean-up filters for increasing the polarization ratio of theoutput beams; and stacking means for stacking said two color sets ontosaid polarization preserving screen such as to enable stereoscopicviewing of the two color sets via orthogonally polarized filters. 26.The apparatus according to claim 25, wherein said optical filter systemincludes, for each projector, a polarization rectifier which transformsa plurality of color components in different polarization states at theinput into the same polarization state at the output by usingexclusively said optical retarders for polarization manipulation. 27.The apparatus according to claim 26, wherein each polarization rectifierincludes: a splitter which separates the color components into twooptical paths, a polarization transformer in at least one optical pathwhich utilizes a said optical retarder to transform the respective colorcomponent to another polarization state in such manner that mutuallyorthogonal polarization states are transformed to polarization statesthat are also mutually orthogonal; and a combiner which combines the twooptical paths for stacking onto said polarization preserving screen. 28.The apparatus according to claim 27, wherein said splitter is a dichroicmirror.
 29. The apparatus according to claim 27, wherein said splitteris a polarization beam splitter.
 30. The apparatus according to claim26, wherein each polarization rectifier includes a stack of said opticalretarders which align the polarizations of all the color components. 31.The apparatus according to claim 30, wherein the color components arered, green and blue and the polarization of the green component isorthogonal to the polarizations of the red and the blue components, andwherein each polarization rectifier includes: a stack of said opticalretarders which rotate the green color component polarization directionby 90° leaving the polarizations of the other color components intact;and a polarization transformer.
 32. The apparatus according to claim 25,wherein said stacking means stacks the images outputted from saidoptical filter system by image warping onto said polarization preservingscreen.
 33. The apparatus according to claim 25, wherein each of saidprojectors is an LCD projector outputting red and blue color componentsin one polarization state, and green color components in an orthogonalpolarization state.
 34. Stereoscopic display apparatus comprising: twoprojection engines having inputs connectable to a source of digital datarepresenting the color components sets of two stereoscopic images, eachof said projection engines having an output outputting an optical beamhaving a set of color components in which at least one color componentof the set is of an orthogonal polarization state with respect to theother color components of the set; a polarization preserving screen; apolarization rectifier for each projection engine effective tomanipulate said polarization states exclusively by optical retarders,and to transform the beams outputted by the projection engines to beamsin which all color components have the same polarization state in such amanner that the two transformed beams have mutually orthogonalpolarizations; a polarization beam splitter for combining thetransformed beams into one co-axial beam; and a projection lens forimaging the stereoscopic images on said screen.
 35. The apparatusaccording to claim 34, wherein each polarization rectifier includes: asplitter which separates the color components into two optical paths, apolarization transformer in at least one optical path which utilizes asaid optical retarder to transform the respective color component toanother polarization state in such manner that mutually orthogonalpolarization states are transformed to polarization states that are alsomutually orthogonal; and a combiner which combines the two optical pathsfor stacking onto said polarization preserving screen.
 36. The apparatusaccording to claim 35, wherein said splitter is a dichroic mirror. 37.The apparatus according to claim 35, wherein said splitter is apolarization beam splitter.
 38. The apparatus according to claim 34,wherein each polarization rectifier includes a stack of said opticalretarders which align the polarizations of all the color components indesired directions.
 39. The apparatus according to claim 38, wherein thecolor components are red, green and blue and the polarization of thegreen component is orthogonal to the polarizations of the red and theblue components, and wherein each polarization rectifier includes: astack of said optical retarders which rotate the green color componentpolarization direction by 90° leaving the polarizations of the othercolor components intact; and a polarization transformer.